In commonly accepted techniques, it is possible to produce hollow metallic products such as the aforementioned sleeve York in a process wherein a blank forged with a shaft at one end thereof, after being cooled, is drilled to form a hollow portion thereof. But, there are some defects in operation efficiency. Accordingly, hot piercing process has been employed. However, in piercing a blank which has an enlarged portion such as a branch of the sleeve York at one end thereof the so called backward extrusion piercing process, in which elongation of the blank occurs in the direction opposed to the piercing punch's forward direction, can not be used because the shape of such an enlarged portion la will be deformed during the piercing, as shown in FIG. 6.
Therefore, for piercing such a blank with an enlarged portion at one end thereof, a forward extrusion piercing process is used in which elongation of the blank occurs in the same direction as the punch's forward direction. However, in a conventional hot piercing process with forward extrusion, one stroke of the piercing punch can not form a through-hole, so that it is needed either (1) to cut off the bottom portion of blank to pass through, after piercing a blank except its bottom portion, or (2) to post-pierce the as yet not pierced bottom portion so as to pass through, after having pierced the blank except at its bottom portion. Thus, the problems described hereunder are accompanied in the process. Otherwise, there is seen Japanese Patent Laid-open No. 243240/1991, the entire disclosure of which is hereby expressly incorporated herein by reference, which discloses an improved forward extrusion piercing process, but this is for piercing cylindrical bodies and thus has different features from those of our invention.
As described above, since the prior art piercing process needs at least two strokes to form a through-hole, operation efficiency is low. Also, since the blank to be pierced is merely located in position on the lower die assembly, an enlarged portion of the blank will be deformed in the piercing process and further, as shown in FIG. 6, any narrow ring portion W will be caused at the stepped portion, or as shown in FIG. 7, (a) and (b), there will be caused a deformation of form Z or narrow ring W at the cylindrical portion of blank.
In particular, in piercing a blank of a slenderness (Pierced length/hole diameter) higher than 4, even prior art piercing can be employed, but it results in wider variation of worked dimensions owing to transverse displacement of the piercing punch. This leads to a low yield and occasionally to breakage or damage of the piercing punch. Accordingly, in the prior art, the piercing process has been used in a range of slenderness of less than 3, and for higher slenderness of blank, machining using a drill should be employed.